10.3 Creating graphical objects

A common feature needed by an application is to have a number of objects displayed in a window and to make events affect the object underneath the cursor. The CAPI provides the ability to create graphical objects, to place them into a window at a specified size and position, and to display them as necessary. Also a function is provided to determine which object is under any given point so that events can be dispatched correctly.

These graphical objects are called
pinboard objects
, as they can only be displayed if they are contained within a
pinboard-layout. To define a pinboard-object, you define a subclass of
drawn-pinboard-object and specify a drawing routine for it (and you can also specify constraints on the size of your object). You can then make instances of these objects and place them into layouts just as if they were ordinary panes. You can also place these objects inside layouts as long as there is a
pinboard-layout
higher up the layout hierarchy that contains the panes.

Note:
pinboard-object
s are implement as graphics on a native window. Compare this with
simple-pane
and its subclasses, where each instance is itself a native window. A consequence of this is that
simple-pane
s do not work well within a
pinboard-layout
, since they always appear above the
pinboard-object
s. For example, to put labels on a pinboard, use
item-pinboard-object
rather than
display-pane
or
title-pane
.

Here is an example of the built-in pinboard object class
item-pinboard-object
which displays its text like a
title-pane
. Note that the function
contain
always creates a
pinboard-layout
as part of the wrapper for the object to be contained, and so it is possible to test the display of
pinboard-object
s in just the same way as you can test other classes of CAPI object.

(contain

;; CONTAIN makes a pinboard-layout if needed, so we don't

;; need one explicitly in this example.

;; You will need an explicit pinboard-layout if you define

;; your own interface class.

(make-instance

'item-pinboard-object

:text "Hello world"))

Figure 10.4 A pinboard object

There is another example illustrating
item-pinboard-object
in the file